Liquid crystal displays (LCD) have become an increasingly important device in today's computer age. LCDs have several distinct advantages over CRTs, including the absence of radiation and substantially lower power consumption. Because of their better brightness and wider viewing angle, active matrix liquid crystal displays (AMLCD) have become the preferred mode of LCDs. An AMLCD generally comprises a plurality of rows and columns of address lines which cross at an angle to one another to thereby form a plurality of crossover points. A pixel is an array element associated with each crossover point which can be selectively addressed. Typically a switching device such as a thin film transistor (TFT) is associated with each array element or pixel. The switching devices allow the individual pixels to be selectively addressed by the application of suitable voltage between respective pairs of row and column address lines, so as to allow images to be shown on the screen.
A TFT generally contains a pair of substantially co-planar source and drain electrodes, a thin film semiconductor material disposed between the source and drain electrodes, and a gate electrode in proximity to the semiconductor but electrically insulated therefrom by a gate insulator. An application of voltage to the gate electrode controls the current flow through the TFT between the pair of source and drain electrodes. This causes the associated pixel of an AMLCD to be switched on and off. Typically, the pixel aperture ratio (i.e., pixel opening relative to the area occupied by a TFT) in an non-overlaying AMLCD is about 50% or less. This causes limited display luminance or large consumption of backlight power. In order to improve battery life, it is highly desirable to increase the pixel aperture ratio. The higher the pixel aperture ratio, the better the display transmission and the longer the batter life for a given luminance.
U.S. Pat. No. 5,780,871, the content thereof is incorporated therein by reference, discloses a high aperture LCD, which contains an array of substantially transparent pixel electrodes which overlap at least one of the address lines to increase the pixel aperture ratios of the LCD. A substantially transparent photo-imageable insulating layer (i.e., photoresist) is disposed between the address lines and the pixel electrodes in the areas of overlap and areas adjacent source electrodes. The photo-imageable insulating layer has a first group of contact vias defined therein by photo-imaging, wherein the pixel electrodes are in electrical communication with corresponding TFT source electrodes through the contact vias of the first defined in the insulating layer.
In theory, it should be possible to apply the device developed in the '871 patent to color LCDs by using a set of color photo-imageable insulating materials. However, attempts to do so have failed, mainly due to the lack of success in finding a suitable color resist which can provide the desired properties, particularly long term stability, for use as a color photo-imageable insulating layer so as to serve as an appropriate color filter.
A good color filter requires good light transmissiblility and color saturation. Both of these properties are closely related to the particle size and uniformity of the coloring materials. Conventionally dyes have been used which satisfy the light transmissiblility and color saturation requirements. However, because of their relatively poor light and heat stability, dyes are not considered desirable coloring material for making color filters. Color filters made using color dyes as the color material have not been shown to provide a long enough useful life.
More recently, pigments, which exhibit substantially improved light and heat resistance, have been used replacing dyes in making color filters. However, in order to achieve satisfactory light transmissiblility and color saturation, the pigments must be very small in size and they also must be homogeneously dispersed in the filter medium. Typically, the particle size of the pigments must be smaller than one-half of the wavelength at which they are expected to operate. The wavelengths of red, green, and blue lights are 610 nm, 540 nm, and 450 nm, respectively. Thus, the particle sizes of the associated pigments must be less than 0.3 .mu.m, 0.27 .mu.m, and 0.23 .mu.m, respectively. Because most pigments, especially organic pigments, have low extent of polarity at their surface and are of relatively high molecular weight molecules, they do not exhibit good affinity with the polymer resin, which makes the bulk of the color filter, or the organic solvent, which is used to coat the polymer resin on the substrate. Both the polymer resin and the organic solvent are relatively highly polar compounds. As a results, the pigments in the color filters tend to coagulate with time and form relatively large particles, causing degradations in the color saturation, color hue, and light transmissibility. This problem becomes even more profound when attempting to use pigments in color resists, which contain further foreign compounds in order to achieve photo imageability.
Because AMLCDs are becoming increasingly important, it is highly desirable to develop a suitable color resist with good color saturation, resolution, light transmissibility, and, more importantly, long term stability, so that it can be advantageously used to provide AMLCDs with increased pixel aperture ratio. Of course, such improved color resist, when developed, will also be very useful in other applications.